The situation before 1st December
Until the changes come into effect, the status quo situation applies - when people import items with a total value including shipping that exceeds the threshold, GST and Customs Duty is calculated and charged by NZ Customs before the items are released for delivery. While this is not strictly based on a dollar figure, effectively any imported books etc with a total value including shipping of NZ$400 or more will incur charges. Actual charges can be calculated HERE. (In actual fact GST and duty if applicable is calculated on all imports, but only charged if it exceeds $60 on a given shipment, and then Custom's entry fees are added). 

Offshore suppliers are not required to collect GST on sold items.

As of 1st December 2019
The main points to know about:

1. Overseas suppliers who have sales to NZ customers totallying $60,000 NZD or more per year will be required to register for GST, charge and collect GST on sold items, and pay that GST to IRD on all sales of NZ$1000 or less*. If you order from one of these suppliers, then you will pay GST on your purchase no matter the size of the order (via the supplier if valued at $1000 or less, and to Customs if over that value).  
2. NZ Customs will now only charge GST and entry fees on shipments totalling more than $1000* at the border.
3. If there is evidence that the GST has already been collected by the off-shore supplier on all items in larger shipments, they will not add more GST at the border (but may add it on items which have not been charged for). 
4. Overseas suppliers who have sales to NZ customers under $60K per year will not be required to register for GST, and should not therefore charge GST on sales. If you order from one of these suppliers, and your total shipment is equal to or less than $1000* NZ, then you will not pay GST, duty or entry fees on those items.
5. Redeliverers, such as NZ Post's YouShop, will be required to charge and collect GST on all items they deliver, irrespective of amount or whether the source company is GST registered. This is because the new legislation requires that goods delivered by a GST-registered redeliverer be treated as if those goods were supplied by the redeliverer. In other words, for example, NZ Post is GST registered in NZ; they must charge GST on the value of items (including shipping) which they collect, ship and deliver to NZ recipients, as well as GST on the cost of their services.
6. If multiple shipments arrive for the same recipient in a short period of time, Customs may combine the totals and treat them as one consignment. (This is unchanged)
7. These rules also apply to second hand books and resources.
8. Non-book items may have additional duty tariffs applicable (eg 5% for toys); if ordering these kinds of items, they will change the way the payable amounts are calculated and charged for shipments over $1000.

*Note! The decision about whether GST is payable on a consignment is made on the value of the consignment itself, exclusive of shipping/packaging/insurance costs. HOWEVER, if it IS payable, then the actual charges are calculated on the total including postage, packaging and insurance (if applicable). This means that overseas suppliers who are required to collect GST will do so on all shipments up to $1000NZ in value, but charge GST on the total including shipping etc. Customs will charge GST etc on all shipments valued at over NZ$1000 not including shipping, but GST is also charged on the cost of shipping.

Example 1: Jill orders $900NZ worth of books from a small company in the U.S, who are not required to register for GST. They therefore don't charge GST. Shipping comes to $125NZ. When the package arrives in NZ, Customs examine the shipping invoice. They see the total value of the items was $900, and therefore do not charge GST, even though with shipping the total would be $1025. 

Example 2: John orders $1100NZ worth of books from a company with less than $60K sales to NZ per year. He pays $1100 plus $150 shipping to the company. Customs see that the total is over $1000, so they charge GST on $1100+$150 = $1250, and also add the entry fee of $55.71. 

Example 3: Kirsty orders $300NZ worth of books from Amazon, with shipping costs of $65NZ. Amazon is registered for GST in NZ. They charge her GST on the cost of the order plus shipping ($365), which they then pay to IRD along with other collected GST on sales.

Example 4: Mark orders from a smaller supplier in the U.S. His shipment includes $300NZ of books, $500NZ of Lego, plastic math manipulatives etc, and $250 in flashcards and educational games, for a total of $1050NZ plus $150 shipping. At the border he would have to pay:
GST= $1200x15%=$180
Duty on $750 of toys @5%=$37.50
Entry fee: $55.71
Total: $273.21

Example 5: Liz orders $100NZ in books and $50NZ worth of "toys" from Amazon. Shipping is $30. She will pay GST on $180 = $27 via Amazon. Amazon (and other overseas suppliers) will not be charging duties on applicable items. 

Example 6: Lisa orders books totalling NZ$950 from Awesome Book Company, who are registered for GST. Shipping costs $100NZ. Awesome Book Company would charge Lisa GST on items plus shipping ($1050). 

For full details on the rules and how they work, see: https://taxpolicy.ird.govt.nz/sites/default/files/2019-sr-gst-low-value-imported-goods.pdf 

1) All products ordered by mail pass through Customs upon arrival in NZ, both for inspection, and for assessment as to whether any duties or GST is due. 
2) Some products attract duty - for example, toys have a 5% duty on them - while others do not. Books have no duty. Nearly all imported products are liable for GST at 15% 
4) GST and Duty are calculated on the TOTAL VALUE OF THE PRODUCTS INCLUDING SHIPPING, converted to NZ dollars at the Customs rate of exchange. Custom's exchange rates are always slightly less favourable than the bank's rates, making the value of the item slightly more than what you may have actually paid for it. 
5) When GST and duty are calculated to be LESS than $60, Customs will waive it and you will not be charged. Their reasoning is that the cost of collection makes it not worth pursuing smaller amounts. 
6) When GST and duty are calculated to be $60 or more, you will be charged for those PLUS a Custom's entry fee of $49.24. You will have to pay the total amount before your parcel is released. 
7) You can use the What's My Duty calculator on the Custom's website HERE to work out what your likely duty, GST and charges will be on items you are considering importing. 

NOTES: 
* If a parcel contains items which have duty on them - eg educational toys - then this could put the total amount over the threshold, and you will be charged that duty, plus GST on everything, plus the entry fee. 
* Do NOT cut the margin too fine! If you place an order for JUST under NZ$400, and rates change, you could end up with additional costs of over $100! I suggest a "margin for error" of at least $20.
* And don't forget shipping/freight charges are included in the "value"  of the parcel!
* NEVER ask a person sending you product from overseas to falsify the value on the parcel - you can held libel for fraud and it can cost you even more!
* If you want items with a value over the threshold, split your orders, BUT try to separate the orders by a couple of weeks or more, as Customs does have the right to combine parcels coming in and charge duty/GST on the combined value. Keep in mind that they can combine totals on multiple parcels from separate sources that arrive at the same time, not just multiple parcels from the same source. 

So let's look at a couple of examples:
1) Family X order various books and curriculum from an American website. The total value of the order, including shipping, in U.S dollars was $253.00

The parcel arrives at Customs. Their exhange rate that day is 0.67, so the NZ value of the parcel is: $253/0.67 = $377.61. There is no duty on the books. The GST on $377.61 = $56.64, less than $60 so there are no charges and the parcel is sent on to the family.

2) Family Y orders curriculum materials from the U.K. The total value of their order, including shipping, is 300 pounds. 

When it arrives in NZ, the Customs U.K exhange rate is 0.54, so the NZ value of the parcel is $300/0.54 = $555.56. Again, there is no duty, but the GST on the parcel comes to $83.33. The family will be charged that plus the entry fee, so a bill totalling $132.57 is sent to them.

3) Family Z orders a combination of books and educational toys from the U.S, which are all sent in one parcel. The total value of the books and shipping is U.S 200, and the value of the toys is U.S $60, for a total of US$260.00

The rate of exhange is 0.67. In this case, Customs will covert the books/shipping value to $298.51. The toys NZ value is $89.55. Together these total less than $400, HOWEVER, the costs add up like this:
Duty on toys @5% = $4.48
GST @15% = $58.21
Entry fee: $49.24
Total to pay: $111.93
This is because the duty and GST combined cross the $60 threshold.

4) Family H orders some ACE materials from SCEE in Australia. I'm giving this example because it's a real one - and it happened last week. The things that happened here are instructive on several points:

​This family ordered curriculum totalling AUD355.02 including shipping. They had worked out that this was less than NZ400 and so should not attract duty and fees. So they were rather upset when they recieved this bill:

Now, in this case, the family were right - they should not have been charged. I was able to speak  to TNT on their behalf and have this bill dismissed, though TNT at first argued with me about it. But I want those seeing this to know these things:

1) Again, allow a margin for error! In this case there was only 60c difference in the GST amount that WOULD have put it over the threshold, and the entire $149.30 above would have been payable. A change in the exhange rate by a point or two would have done it. 
2) This bill was issued by TNT, not Customs. When companies ship goods from overseas via carriers other than the standard mail, or using airway bills, then their handlers at this end do their own calculations of things and collect money on behalf of customs. In this case, you can see that they then also add their own additional fees!
3) TNT was operating on the erroneous assumption that since GST plus the entry fee came to over $60, they should charge. Customs entry fee is NOT included in the $60 threshold (if it were, you could expect to be charged on all parcels over about NZ$70 in value!)
4) If this hadn't been paid or sorted out within 3 business days, TNT were going to add extra charges of $20/day for "storage"!

So, if you're ordering from SCEE, be extra careful your parcel will come in well under the NZ$400 threshold, including shipping or you'll get extra charges upon extra charges! Watch this, or consider an alternative source of Paces.